System and vessel for hydrocarbon production and method for intervention on subsea equipment

ABSTRACT

A system for operation and service of at least one hydrocarbon-producing well and of equipment, which is disposed under water, for further transport of the well stream, where the system comprises at least one pipeline, at least one booster unit, a vessel comprising an anchoring system about which the vessel can rotate or swivel, means for conducting an intervention on subsea equipment, means for communication with and control of the at least one well and other equipment, means for injecting chemicals into the well streams, means for transferring to and inserting pigs in the pipeline, means for production of energy for operation of subsea equipment, means for transferring energy from the means for production of energy to the booster unit and any other subsea equipment.

The present patent application relates to a system for operation andservice of hydrocarbon-producing wells, a vessel for use in operationand maintenance of hydrocarbon-producing wells and a method forintervention on subsea equipment which is in use in connection withproduction and further transport of a well stream.

The principle is known of producing oil and gas from a ship lyinganchored/DP-positioned over a subsea field of hydrocarbons. Until nowthe main task of this ship has been to receive, process and store thewell stream. The unprocessed well stream is processed and electric powerand other energy required for running the process is generated on boardthe ship. In recent years systems have been developed where parts of theprocessing equipment are located on the seabed, for example multiphasepumps and water injection pumps. In this case, electric power is passedin a riser/umbilical system from the ship down to the bottom-mountedequipment. Where environmentally vulnerable areas are involved, it isdesirable to reduce the environmental exposure to a minimum during theproduction of oil and gas. Concepts are therefore being considered whichare based on transportation of unprocessed well stream in pipelines fromthe field to a land-based plant or processing plant on other adjacentplatforms or floating platforms for further processing. The principalchallenge with concepts of this type appears to be the actual pipetransport (flow assurance problems) and operation of pressure boosterequipment such as multiphase pumps and wet gas compressors withassociated electric power system. When the field and the land-basedplant are at a great distance apart, offshore power generation seems tobe the most attractive solution.

From the prior art, in U.S. Pat. No. 6,2363,971 B1 a system is disclosedfor subsea production of hydrocarbons where an intermediate platform isarranged between the seabed and the water surface and a floatingplatform on the surface. The well stream is taken up to the intermediateplatform on which are mounted various pieces of equipment includingpumping equipment and equipment for measuring volume flow. Gas from theproduced well stream can be separated and taken up to the floatingplatform where it is used for production of electrical energy, while ariser connected to the intermediate platform transports the rest of thewell stream to a processing destination near the hydrocarbon field.

In GB 2 292 760 A a production vessel is disclosed, the aft end of whichis provided with a device for receiving a buoy, and round which thevessel can pivot, the vessel being anchored to the bottom. The riserstransporting the well stream are connected to the buoy. At the other endof the vessel a rig is provided for use in maintenance of the productionwell.

U.S. Pat. No. 6,364,021 B1 discloses a system comprising a riser fromthe wellhead to a floating buoy on the ocean surface. The system furthercomprises a vessel which can go alongside the buoy and conductpredetermined maintenance operations.

The proposed system seeks to provide an integrated system for productionof hydrocarbons from subsea wells which will have a number of advantageswhen compared with the prior art.

Since the hydrocarbons will not be processed on the vessel, the normalprocessing equipment will be superfluous, and this results in a numberof advantages. Since the vessel will not be provided with processingequipment, the usual “rules and regulations” applying to productionvessels are avoided, thereby making the vessel cheaper to build. Whenthe vessel has no processing equipment on board, space is also madeavailable for storing, for example, equipment for use in intervention onsubsea equipment, spare parts, pigs and equipment for launching pigs,chemicals for injecting into the well stream, fuel for energyproduction, etc. The fact that the well stream is not taken up to thevessel for processing also reduces the environmental risk associatedwith production, an aspect of offshore production of hydrocarbons whichwill become increasingly important in the future. Another advantage ofthe system is the possibility it offers of simultaneous intervention andproduction, which may be important for maintaining flow and temperaturein the transport pipe.

An object of the invention, therefore, is to provide a system foroffshore production of hydrocarbons where the risk of accidentsresulting in environmental damage is reduced.

Another object of the invention is to provide a system which permits thecombination of power generation and intervention on subseainstallations.

A further object of the present invention is to provide a system foroffshore production of hydrocarbons with improved assurance ofuninterrupted production of hydrocarbons, and particularly assurancethat the transport system for the hydrocarbons is in continuousoperation.

It is also an object of the invention to provide a system which resultsin simplified operation and maintenance of subsea equipment, such aswell, wellhead, pressure booster equipment and other equipment disposedbelow the surface of the water.

This is achieved by means of a system according to claim 1, a vesselaccording to claim 16 and a method according to claim 22. Furtherpreferred embodiments of the invention are indicated in the dependentclaims.

A system is provided for operation and service of at least onehydrocarbon-producing well, and of equipment for further transport ofthe well stream and any equipment required for treatment of the wellstream. The equipment and the at least one well are disposed below awater surface, preferably on a seabed or an ocean floor. The systemfurther comprises at least one pipeline for transport of the well streamto the desired destination, and at least one pressure booster unit fordriving the well stream through the at least one pipeline, a vessel foroperation and service of the at least one well and the equipmentdisposed below the water surface, where the vessel comprises means forcontrolling operating and service functions, together with means forconducting an intervention on equipment disposed below the watersurface. The system further comprises means for production of energy foroperation of the equipment disposed below the water surface, as well asmeans for transferring energy from the means for production of energy toat least the at least one pressure booster unit which drives the wellstream of hydrocarbons through the at least one pipeline and means forcommunication with and control of the at least one well and theequipment disposed below the water surface.

The system may also comprise two or more hydrocarbon-producing wells. Inthis case a pressure booster unit may be mounted at each well or thesystem may be provided with a pressure booster station where the wellstreams from the hydrocarbon-producing wells are gathered andpressurised. From the pressure booster station at least one pipeline isprovided for further transport of the well streams to the desireddestination.

In a preferred embodiment the vessel is provided with an anchoringsystem comprising a mooring system, a dynamic positioning system or acombination of the two.

The anchoring system will advantageously be arranged in such a mannerthat the vessel can rotate about the anchor point on the vessel orswivel around the anchoring system when the vessel is anchored via oneor more mooring lines to an anchored buoy. The buoy may be arranged insuch a manner that it is lying on the surface of the water or it may bearranged so that it is located between the surface and the bottom. Thevessel will then be able to pivot round the buoy. It will also bepossible to fasten the vessel directly to an anchor on the seabed orocean floor, in which case the vessel will be able to pivot round thispoint. When the vessel is anchored to a buoy, risers or umbilicals maybe arranged in such a manner that they either extend from the vessel tothe buoy and from there on to a swivel device at the anchor on thebottom, or they may extend directly from the vessel down to the swiveldevice at the anchor on the bottom.

When one or more adjacent wells are developed, it will be advantageousto place essential equipment, which has to be disposed below the watersurface, under or within the curve described by the vessel's outer pointas the vessel rotates about or swivels around the anchoring system, i.e.the vessel rotates about an anchor point in the vessel or pivots arounda buoy to which the vessel is anchored. This will make it easy toposition the vessel when maintenance or repairs have to be carried outon equipment located below the surface of the water, and preferably onthe seabed or ocean floor.

The vessel is advantageously arranged so that it can be disconnectedfrom its anchoring system and riser and umbilical system for positioningover the intervention object on the seabed or ocean floor if this objectis not located within the above-mentioned curve. This will also beadvantageous in bad weather when it may be necessary to perform a rapiddisconnection of the anchoring system and riser and umbilical system.

The means for production of energy for operation of equipment disposedbelow the water surface may either be mounted in its entirety on thevessel or in its entirety below the water surface. Parts of the meansfor energy production may possibly be mounted on the vessel, while therest of the means are mounted below the water surface.

In order to provide energy, the system may be provided with means belowthe surface of the water, preferably on the seabed or ocean floor inconnection with the well or the at least one pipeline, in order toremove a part of the gas in the well stream together with means fortransporting the gas up to the vessel. The transport of gas up to theplatform may be carried out by means of a separate pipeline or throughan umbilical, which is also used for other purposes in connection withthe operation of the well.

One of the advantages of not taking the well stream of hydrocarbons upto the vessel for processing is that the vessel is not encumbered withall the processing equipment with which the production vessels arenormally equipped today. This absence of processing equipment forprocessing the well stream makes a great deal of space available on thevessel. The vessel is therefore provided with storage capacity fornecessary contributory factors in connection with operation andmaintenance of the system, such as chemicals for injecting into the wellstream, pigs and equipment for launching and receiving pigs, spare partsand equipment for use in intervention on equipment below the watersurface and possibly fuel required for energy production.

In order to ensure that the stream of hydrocarbons in the at least onepipeline does not come to a stop, the system may be provided with meansfor transferring chemicals from the vessel to the at least one pipeline.These are chemicals which are injected in order to prevent the formationof wax and hydrate in the pipes and they can be transferred from thevessel by means of a separate pipeline or through an umbilical which isalso used for other purposes in connection with the operation of thewell or wells. The system is also provided with necessary equipment suchas valves for injecting the chemicals into the well stream in thepipeline.

The system will also advantageously comprise means for launching andreceiving pigs in the at least one pipeline and means for transferringpigs down from the vessel to the at least one pipeline or from the atleast one pipeline to the vessel. These may be pigs for scraping andcleaning the at least one pipeline in order to avoid shutdown or theymay be intelligent pigs that are passed through the at least onepipeline for inspection of the pipeline's integrity.

The system may also advantageously comprise heating means for heatingthe at least one pipeline as an additional preventive measure againststoppage of the production of hydrocarbons, as well as means fortransferring energy from the vessel required for the heating meansprovided in connection with the at least one pipeline. The energytransferred from the vessel to the at least one pipeline will preferablybe electrical energy, and the means for transferring the energy from thevessel to the at least one pipeline will therefore be an electric cable.This may be a separate cable or it may form a part of an umbilical.These heating means for the at least one pipeline may extend over ashorter or longer distance along the pipeline which transports thehydrocarbons to the desired destination where processing will takeplace. The heating means may also be arranged along the entire length ofthe pipeline.

In addition to the equipment already mentioned above, the system mayalso include additional equipment for treatment of the well stream,disposed below the surface of the water, and particularly on the seabedor ocean floor. This may include equipment for mixing, desanding andwater separation.

Furthermore, it will be necessary to monitor the production ofhydrocarbons and the rest of the system. The system will thereforeadvantageously include means for communicating production and controldata from the at least one well and other equipment, which is disposedbelow the water surface, to the vessel. Similarly, the system willadvantageously include means for transferring control signals from thevessel to the at least one well and other equipment disposed below thesurface of the water. Such signals will normally be transmitted throughan umbilical containing both electric cables for electrical signals andpipes for hydraulic or pneumatic signals.

A vessel is also provided for use in connection with operation andmaintenance of a system for production of a well stream containinghydrocarbons from at least one well, where the at least one well and theequipment are disposed below a water surface, particularly on a seabedor an ocean floor. The vessel is provided with a system for anchoring,e.g. a mooring system, a dynamic positioning system or a combination ofthe two.

Furthermore, the vessel may advantageously comprise means for productionof energy for operation of equipment disposed below the water surface,and also possibly for heating at least one pipeline disposed below thewater surface.

Moreover, the vessel may advantageously comprise means for transferringchemicals to at least one pipeline disposed below the water surface. Thechemicals are added to the well stream in order to prevent hydrateformation and they can be transferred from the vessel to the at leastone pipeline through a separate pipeline or through an umbilical whichcan also be used for other purposes in connection with the production ofthe hydrocarbons.

Moreover, the vessel may advantageously comprise storage space forstoring the chemicals which are to be injected into the pipeline asexplained above.

Furthermore, the vessel may advantageously comprise means for carryingout an intervention on equipment disposed below the water surface. Thesemeans include tool systems which can be lowered through an opening inthe vessel or from a lifting arrangement which is normally located atthe after end of the vessel. The anchor point and location of theequipment for lowering and raising the tool systems will normally belocated in different parts of the vessel so that the vessel covers agreater area when it rotates or pivots round the anchor. The tool systemused in a given situation will be dependent on what kind of equipment isto be worked on and which operations have to be performed on theequipment.

Furthermore, the vessel may advantageously comprise storage space forstoring tool systems and equipment used during intervention on equipmentbelow the water surface and spare parts for said equipment below thewater surface. In particular this will be parts for equipment which iscritical for the production of hydrocarbons and for ensuring that thewell stream can flow uninterruptedly through the at least one pipeline.

Furthermore, the vessel may advantageously comprise means fortransferring pigs to a pipeline disposed below the water surface. A pigis transferred from the vessel to a subsea receiving station either in atool which is lowered and connected to the at least one pipeline or in aflexible transfer riser which may be secured to the at least onepipeline's pig launching system which can be hoisted up to the vessel asrequired. The pig is installed in a controlled environment on the vesseland passed through the flexible riser in a controlled manner down to thepig receiving system. It is expedient to pass both sophisticatedinstrument pigs through the pipe in order to monitor the pipe'sintegrity and simpler scraper pigs to remove wax, sand, water, etc. thathave been deposited in the pipe.

Furthermore, the vessel may advantageously comprise storage space forstoring pigs and equipment for transferring pigs to the pipeline.

The anchoring system may be arranged in such a manner that the vesselrotates about an anchor body on the vessel. Anchor lines extend from theanchor body for anchoring the anchor body and thereby also the vessel.Alternatively, the vessel may swivel around the anchoring system by thevessel being anchored via a mooring line to an anchored buoy. The vesselwill then be able to pivot round the buoy. It will also be possible tofasten the vessel directly to an anchor on the seabed or ocean floor, inwhich case the vessel will be able to pivot round this point.

The vessel will advantageously be arranged in such a manner that it canbe disconnected from the anchor as well as communication line(s),umbilical(s) and other devices connecting the vessel to the at least onewell and the equipment disposed below the surface of the water. Asmentioned above, this will be particularly advantageous where badweather, possibly combined with a mishap or an accident, make itnecessary for the said equipment to be disconnected from the vessel.

The means for producing energy on board the vessel may consist of atleast one generator set. This may typically comprise a diesel engine ora gas turbine driving an electric generator. In this case the vesselwill advantageously also include storage space for necessary fuel suchas diesel or gas for operation of the at least one generator set. Thevessel may advantageously be provided with two or more generator sets sothat the vessel has at least one spare set in case a set breaks down.

The means for producing energy for operation of the at least one welland equipment disposed below the water surface may also comprise a smallprocessing plant. In this case an appropriate amount of gas will beremoved from the well stream and transferred to the vessel forprocessing. The vessel will then also be provided with at least onegenerator set for production of energy from the processed gas. As in theabove case, the vessel will advantageously be provided with two or moregenerator sets in case operational problems should arise with one of thesets.

The vessel is furthermore advantageously provided with means fortransferring the produced energy from the vessel to subsea equipment andpossibly the at least one pipeline. This may involve a separate electriccable or an electric cable which is a part of an umbilical. If theenergy transferred is in the form of hydraulic or pneumatic energy,these means could be a separate pipeline or a pipeline which is a partof an umbilical.

As mentioned earlier, parts of or all the equipment for energyproduction may be provided below the water surface instead of on thevessel.

A method is also provided for intervention on equipment in use inconnection with production of hydrocarbons from at least one well andequipment for further transport of the well stream, where said equipmentis disposed below a water surface, particularly on a seabed or oceanfloor. The intervention is conducted from a vessel which is speciallyarranged for operation and maintenance of equipment disposed below thewater surface. The vessel is provided with a system for anchoring, e.g.a mooring system, a dynamic positioning system or a combination of thetwo, where the anchoring system is arranged in such a manner that thevessel can rotate about or swivel round the anchoring system. The vesselis further provided with means for conducting an intervention on one ormore intervention objects disposed below the water surface andpreferably on the seabed or ocean floor, and means for loweringintervention equipment and possibly spare parts down to the interventionobject or intervention objects. The method for conducting theintervention consists of the following steps:

-   -   the operating and service vessel is positioned in such a manner        that means for lowering intervention equipment and any spare        parts are located substantially above the intervention object,    -   the operating and service vessel may be disconnected from its        anchoring system as well as communication line(s), umbilical(s)        and other devices connecting the vessel to one or more wells and        equipment disposed below the water surface, thus enabling it to        be positioned over the intervention object,    -   the intervention equipment and any spare parts are lowered down        to the intervention object,    -   necessary service or repair is carried out on the intervention        object,    -   if necessary, both service and repair are carried out on the        intervention object,    -   the intervention equipment is lifted up to the operating and        service vessel.

When the operating and service vessel is positioned for loweringnecessary tool systems and any spare parts for carrying out anintervention, underwater currents, for example, may make it necessary toposition the vessel slightly to the side instead of being locatedvertically directly above the intervention object. The term“substantially above” should therefore be understood to mean that thevessel may be located slightly to the side relative to a vertical linefrom the intervention object if so dictated by the existing conditions.

An embodiment of the invention will now be described with reference tothe figures, in which:

FIG. 1 illustrates the system with some of its elements,

FIGS. 2-4 illustrate alternative methods of anchoring the vessel,

FIG. 5 illustrates an intervention situation on equipment located belowthe surface of the water,

FIG. 6 illustrates an example of how equipment for launching pigs can bearranged.

FIG. 1 is a general schematic view including most of the system's maincomponents. A vessel 10 or other floating structure is kept in positionby means of an anchoring system where anchor lines 11 are attached to alower part of the swivel system 13 mounted on the vessel 10, thusenabling the vessel to rotate about the anchoring system. At their otherend the anchor lines are secured by the anchors 12. The vessel orfloating structure 10 may also be anchored by alternative means, whichwill be explained below.

The ship is positioned in a suitable position relative to a subseaproduction system for oil and gas. The production system may beorganised with one or more wells 24 with templates 34 with at least oneintegrated pressure booster unit 27 on each template or the well streams25 may be collected in a central pressure booster station 35, where thepressure boosting is provided by at least one pressure booster unit 27,comprising one or more pumps or compressors. If necessary, several suchpressure booster stations 35 may be provided. At the front edge of theat least one pressure booster unit 27, pre-processing means 26 may beprovided for pre-processing the well stream and in the same way at theafter edge of the at least one pressure booster unit 27, post-processingmeans 28 may be provided for post-processing the well stream. These pre-and post-processing means may, for example, be mixers, desandingequipment, water separation equipment and chemical injection equipment.The bottom-mounted equipment communicates with the ship through one ormore risers 20 or umbilicals 21.

The vessel 10 is provided with at least one suitable power generatingunit 14 which produces electric power and delivers it through thevessel's swivel system 13 and on through an electrical umbilical 22 to atermination point 30. If necessary, several electrical umbilicals 22 maybe installed. At least one power regulating unit 15 may either bemounted on the vessel in its entirety, or the whole or parts of this atleast one unit 15 may be mounted underwater in connection withassociated power consumers such as pumps and compressors.

For operation of the vessel's at least one power generating unit 14, twopossible solutions are suggested. The vessel 10 may carry the necessaryfuel, such as, e.g., diesel, stored in separate fuel tanks 16 on thevessel. These tanks 16 may be refilled from other ships through aconventional transfer system. Alternatively, a small portion of theprocess flow, removed from the gas phase, may be passed up to the vessel10 through a suitable riser 23. A compact little processing plant 36pre-processes this gas flow so that it can be used as fuel in the powergenerating unit 14.

A processing plant of this kind may also be provided under water,preferably on the seabed. Parts of the processing plant may also beprovided on the vessel while the rest is mounted under water. In orderto ensure regularity and fuel during start-up and shutdown phases, itmay also be expedient to have a compact little intermediate store 19 forgas. Alternatively, operating conditions of this kind may be met bysupplying the power generating unit 14 with fuel from the ship's tanks16. It may also be expedient to mount parts of or all the powergenerating equipment under water if public regulations or otherlegislation dictate that it is necessary.

In FIGS. 2-4 various alternative methods are indicated for anchoring thevessel 10. In general the vessel or floating structure can be anchoredby means of a mooring system or a dynamic positioning system or acombination of the two. In FIG. 2 the vessel is moored to a floatingbuoy 40 via a mooring line 47 which is connected to the vessel and thebuoy by suitable connections 41, 42. The buoy is secured via at leastone anchor line 48 to a swivel device 44 mounted on an anchor 45. One ormore risers 49 and one or more umbilicals 46 pass from the ship down tothe swivel 44 on the seabed and continue from there to subsea equipment(not shown in the figure). In FIG. 3 an anchoring method is illustratedwhere riser 50 and umbilicals 51 together with one or more anchor lines52 extend directly from the vessel 10 down to a swivel device 44connected to an anchor 45. The riser and umbilical system continues to,for example, a pressure booster station 35 or other subsea equipment. InFIG. 4 a vessel 10 is illustrated where riser 56 and umbilical(s) 57together with one or more mooring lines 59 are connected to a buoy 55.The buoy is secured via one or more anchor lines 58 to a swivel device44 connected to an anchor 45.

Riser 56 and umbilical(s) 57 continue from the buoy 55 to the swivel 44and from there on to subsea equipment such as a pressure booster station35. In FIGS. 2 and 4 the buoys 40, 55 are shown floating on the surfaceof the water. These buoys 40, 55 may also be arranged in a submergedposition, located between the water surface and the seabed or oceanfloor, thereby providing a simpler riser system.

The vessel 10 may be positioned in such a manner that by pivoting thevessel to given positions it can be employed as an intervention platformfor subsea-mounted equipment such as, for example, a pressure boosterstation 35, i.e. the at least one pressure booster unit, pre- andpost-processing means 26, 28, etc. as illustrated in FIG. 5.Alternatively, the vessel 10 may be disconnected from its anchoring andriser system and positioned freely over the intervention object. Whenthe vessel 10 is in position, relevant tool systems 62 (FIG. 5) arelowered in the conventional manner to subsea equipment 63, 64 from thevessel through either an opening 60 in the vessel or from a liftingarrangement 61 which will normally be positioned in the after end of thevessel 10. When the vessel 10 is positioned without disconnection, inmany cases the intervention system may be arranged so that the vesselcan continue to deliver electric power and other contributory factors toparts of the production system not affected by the ongoing, simultaneousintervention operation.

An important operation which can be conducted from the vessel 10 will beinstallation and operation of a subsea system for launching pigs. A pigis transferred from the vessel 10 to its subsea receiving station eitherin a tool 73 (FIG. 6) which is connected to the pipe system through asubsea station 71, or in a flexible pig transport pipe (not shown in thefigure) which is secured to the pipe system's pig launching system, andis hoisted up to the vessel when required. The flexible pig transportpipe may be laid on the seabed or suspended under the vessel in asuitable manner when not in use. The pig is installed in a controlledfashion on the vessel and passed in a controlled fashion down throughthe riser to the pig receiving system. It may be expedient to pass bothsophisticated instrument pigs through the pipe in order to monitor thepipe's integrity and simpler scraper pigs in order to remove wax, sand,water, etc. which have been deposited in the pipe. Equipment, pigs andany spare parts used in connection with pigging and other interventionon the subsea installations in the system may be stored in suitablestorage spaces 18 (FIG. 1) on board the vessel 10.

In addition to power generation and monitoring functions, in most casesit will be desirable to add chemicals to the well stream in order tosafeguard the integrity of the pipe and an efficient flow (corrosion,wax and/or hydrate inhibitors) in the pipe system 32 to shore 33 wherefurther processing will take place. These chemicals will advantageouslybe able to be stored in separate tanks 17 (FIG. 1) on the vessel 10 andtransferred to the subsea equipment through the umbilical system. Byinjecting the chemicals on the suction side of the pressure booster unit27 (the pump(s)/compressor(s)), very good distribution of the fluidvolume will be obtained, i.e. good mixing.

It may also be expedient, either under special conditions (for exampleat start-up), or on a permanent basis, to supply electric power throughat least one heating means 31 (FIG. 1) for direct heating of thepipeline system, from the well or other equipment located on the seabedor ocean floor and some distance towards land 33 or possibly all the wayto land 33. Such a system may also be employed together with pipemonitoring systems, thereby enabling the vessel 10 to regulate in anefficient manner processing equipment and injection equipment accordingto the state of the transport system (pipe).

The vessel 10 will also be a platform for necessary monitoring andcontrol systems for parts of, or preferably all subsea-mountedequipment. This may be operated from onshore or locally from the vessel.

1. A system for operation and service of at least onehydrocarbon-producing well and of equipment for further transport of thewell stream and any equipment for treatment of the well stream, wheresaid equipment and at least one well are disposed below a water surface,particularly on a seabed or an ocean floor, which system comprises: atleast one pipeline for transport of the well stream to the desireddestination, at least one pressure booster unit for driveling the wellstream through the at least one pipeline, wherein the system furthercomprises a vessel for operation and service of the at least one welland the equipment disposed below the water surface, which vesselcomprises an anchoring system comprising a mooring system, a dynamicpositioning system or a combination of the two, and where the anchoringsystem is arranged in such a manner that the vessel can rotate about orswivel around the anchoring system, means for conducting an interventionon equipment disposed below the water surface, means for communicationwith and control of the at least one well and other equipment disposedbelow the water surface, means for transferring chemicals from thevessel to the well streams and the at least one pipeline and means forinjecting the chemicals into the well stream and the well stream in thepipeline, means for inserting pigs in the at least one pipeline andmeans for transferring pigs from the vessel to the at least onepipeline, means for production of energy for operation of equipmentdisposed below the water surface, means for transferring energy from themeans for production of energy to the at least one pressure booster unitand any other subsea equipment.
 2. A system according to claim 1,wherein at least one pressure booster unit is mounted at each well.
 3. Asystem according to claim 1, wherein the system comprises a pressurebooster station where the well streams from the hydrocarbon-producingwells are collected and pressurised and from which at least one pipelineis provided for further transport of the well streams to the desireddestination.
 4. A system according to claim 1, wherein equipmentdisposed below the water surface is preferably arranged below and withinthe curve described by the vessel's outer point as the vessel rotatesabout or swivels around the anchoring system.
 5. A system according toclaim 1, wherein the means for production of energy for operation ofequipment disposed below the water surface is either provided in itsentirety on the vessel or in its entirety below the water surface.
 6. Asystem according to claim 1, wherein parts of the means for productionof energy for operation of equipment disposed below the water surface isprovided on the vessel, while the rest of the means are provided belowthe water surface.
 7. A system according to claim 1, wherein the systemis provided with means for removing a part of the gas in the well streamas well as means for transporting the gas to means for production ofenergy.
 8. A system according to claim 7, wherein the means forproducing the energy for operation of equipment and the at least onewell which are disposed below the water surface also comprise aprocessing plant adapted for processing the gas removed from the wellstream for operation of the power generating unit.
 9. A system accordingto claim 1, wherein the vessel is provided with storage capacity fornecessary contributing factors in connection with operation andmaintenance of the system, such as chemicals for injecting into the wellstream, pigs and equipment for launching and receiving pigs, spare partsand equipment for use in intervention on equipment below the watersurface, and possibly fuel required for energy production.
 10. A systemaccording to claim 1, wherein the system comprises heating means (forheating the at least one pipeline, and means (for transferring necessaryenergy from the means for production of energy to the heating means. 11.A system according to claim 1, wherein the system also comprisesadditional equipment for treatment of the well stream disposed below thewater surface, and particularly on the seabed or ocean floor, such asequipment for mixing, desanding and water separation.
 12. A systemaccording to claim 1, wherein the system comprises means forcommunicating production and control data from the at least one well andother equipment, which is disposed below the water surface, to thevessel.
 13. A system according to claim 1, wherein the system comprisesmeans for transmitting control signals from the vessel to the at leastone well and other equipment disposed below the water surface.
 14. Avessel for operation and maintenance in connection with production of awell stream containing hydrocarbons from at least one well andmaintenance of production equipment and equipment for further transportof the well stream, where the at least one well and the equipment aredisposed below the water surface, particularly on a seabed or an oceanfloor, wherein the vessel further comprises: a system for anchoringcomprising a mooring system or a dynamic positioning system or acombination of the two, which anchor is arranged in such a manner thatthe vessel can rotate about or swivel around the said anchor, means fortransferring chemicals to at least one pipeline disposed below the watersurface, storage space for storing chemicals that are to be injectedinto the pipeline, means for conducting an intervention on equipmentdisposed below the water surface, storage space for storing equipmentfor use in intervention on equipment below the water surface and spareparts for said equipment below the water surface, means for transferringpigs to a pipeline disposed below the water surface, storage space forstoring pigs and equipment for transferring pigs to the pipeline.
 15. Avessel according to claim 14, wherein the vessel is arranged in such amanner that it can be disconnected from the anchor as well ascommunication line(s), umbilical(s) and other devices connecting thevessel to the at least one well and the equipment disposed below thewater surface.
 16. A vessel according to, claim 14, wherein the vesselcomprises means for production of energy for operation of equipmentdisposed below the water surface, as well as any heating of at least onepipeline disposed below the water surface.
 17. A vessel according toclaim 16, wherein a part of the means for production of energy isprovided on the vessel, while the rest of the means are provided belowthe water surface.
 18. A vessel according to, claim 14, wherein themeans for producing energy comprise at least one generator set.
 19. Avessel according to claim 18, wherein the vessel comprises fuel tanksfor fuel required for operation of the at least one generator set.
 20. Avessel according to claim 14, wherein the vessel comprises means fortransferring the produced energy from the vessel to subsea equipment andpossibly the at least one pipeline.
 21. A method for intervention onequipment in use in connection with production of a well streamcontaining hydrocarbons from at least one well and equipment for furthertransport of the well stream, where said equipment is disposed below awater surface, particularly on a seabed or ocean floor, and where theintervention is conducted from a vessel for operation and maintenance ofequipment disposed below the water surface, where the vessel is arrangedin such a manner that it can rotate about or swivel round its anchoringsystem, and which vessel comprises means for conducting an interventionon subsea equipment disposed below the water surface, and means forlowering a tool system and any spare parts down to the subsea equipment,wherein the operating and service vessel is positioned in such a mannerthat means for lowering a tool system and any spare parts are locatedsubstantially over the subsea equipment, the operating and servicevessel may be disconnected from its anchoring system as well ascommunication line(s), umbilical(s) and other devices connecting thevessel to one or more wells and equipment disposed below the watersurface, thus enabling it to be positioned over the subsea equipment,the tool system and any spare parts are lowered down to the subseaequipment, necessary service or repair is carried out on the subseaequipment, if necessary, both service and repair are carried out on thesubsea equipment, the tool system is lifted up to the operating andservice vessel.